The present invention relates to an active matrix substrate having formed thereon a matrix of non-linear elements serving as switching elements, such as thin film transistors, and to a producing method of the same.
In a general liquid crystal display element, a display pattern is formed on the screen by selectively driving a matrix of pixel electrodes. In other words, in the above liquid crystal display element, when a voltage is applied across a selected pixel electrode and an opposing electrode, a liquid crystal interposed between these two electrodes as a display medium is optically modulated, and such an optical modulation is recognized as a display pattern.
The active matrix driving method is known as a driving method of the above pixel electrodes. In this method, a matrix of independent pixel electrodes are connected to their respective switching elements, so that each pixel electrode is driven by the ON/OFF action of the switching element. An example of the switching element is a non-linear element, such as a thin film transistor (hereinafter, referred to as TFT), an MIM (metal insulator metal) element, a MOS (metal oxide semiconductor) transistor element, and a diode.
As shown in FIG. 15 as an example, an active matrix substrate using TFTs as the switching elements is arranged in such a manner that a plurality of parallel scanning lines 104 are provided to intersect at right angles with a plurality of parallel signal lines 105.
A pixel electrode 102 is provided to each rectangular area enclosed by the scanning lines 104 and signal lines 105. Also, a TFT 101 functioning as the switching element is provided in the vicinity of each intersection of the scanning lines 104 and signal lines 105.
Each TFT 101 comprises a gate electrode 101g connected to the scanning line 104 electrically, a source electrode 101s connected to the signal line 105 electrically, and a drain electrode 101d connected to the pixel electrode 102 electrically.
The switching element like the TFT 101 is produced by repeating the film forming and etching steps of a conductor layer, a semiconductor layer, and an insulating layer. Thus, a static electricity is often generated during the producing process or transportation process from one apparatus to another, the switching elements formed on the substrate are susceptible to the static-induced damage.
To solve the above problem, various methods have been proposed to protect the switching elements and the like from the static electricity generated during the producing process.
For example, a method disclosed in Japanese Laid-open Patent Application No. 106788/1988 (Tokukaisho No. 63-106788) is illustrated in FIG. 16, in which a conductor short-ring 108 is provided to interconnect all the input terminals electrically. To be more specific, scanning line input terminals 106 connected to the scanning lines 104 in an active matrix portion 103 and signal line input terminals 107 connected to the signal lines 105 in the active matrix portion 103 are interconnected electrically through the conductor short-ring 108. According to this arrangement, a static electricity inputted into any of the scanning line input terminals 106 and signal line input terminals 107 can be dispersed to all the other input terminals 106 and 107 through the conductor short-ring 108.
In other words, when a static electricity is inputted one of the scanning line input terminals 106 and signal line input terminals 107, the input static electricity is dispersed to all the other input terminals 106 and 107 through the conductor short-ring 108 interconnecting these input terminals 106 and 107 electrically. Thus, if a static electricity is inputted into one of the scanning line input terminals 106, the switching elements 101 and pixel electrodes 102 connected to the corresponding scanning line 104 are not affected by the input static electricity.
However, if the input terminals are interconnected through the conductor short-ring 108 as shown in FIG. 16, the conductor short-ring 108 must be removed before a driver is mounted to each input terminal. Therefore, there is no static electricity preventing means in the steps after the driver is mounted, and the switching elements and the like formed on the substrate may be damaged by the static electricity.
To solve the above problem, the above reference discloses an other method of preventing the switching elements and the like from the static electricity generated during the producing process, which is illustrated in FIG. 17. More specifically, the scanning lines 104 and signal lines 105 between the active matrix portion 103 and the scanning line input terminals 106/signal line input terminals 107 are interconnected electrically through a semiconductor short-ring 109 of high resistance made of a semiconductor having a high resistance. According to this arrangement, a static electricity inputted into any of the scanning line input terminals 106 and signal line input terminals 107 can be dispersed to all the other input terminals 106 and 107.
In other words, when a static electricity is inputted into any of the scanning line input terminals 106 and signal line input terminals 107, the input static electricity is dispersed to all the other input terminals 106 and 107 by means of the semiconductor short-ring 109 of high resistance through the scanning lines 104 and signal lines 105.
When all the lines are interconnected through the semiconductor short-ring 109 of high resistance in the above manner, it is not necessary to remove the semiconductor short-ring 109 of high resistance before the driver is mounted to each input terminal. Consequently, the static-induced damage to the switching elements is prevented in the steps not only before but also after the driver is mounted.
However, when the active matrix substrate uses the above semiconductor short-ring 109 of high resistance, it becomes quite difficult to stabilize a resistance value of the semiconductor layer during the producing process, and a problem occurs if the resistance value of the semiconductor short-ring 109 of high resistance is not set to an adequate value. That is, when the resistance value of the semiconductor short-ring 109 of high resistance is too small, there occurs a serious defect, namely, leakage between the input terminals. On the other hand, when the resistance value of the semiconductor short-ring 109 of high resistance is too large, the semiconductor short-ring 109 of high resistance can not function as a short-ring.
Further, in the method of using the semiconductor layer as the short-ring, if a channel etch type TFT is used as the switching element of the active matrix portion 103, the semiconductor layer which will be made into the short-ring must be masked by a photoresist when the semiconductor layer is produced concurrently with the TFT.
Therefore, this method can not adopt a short-cut process, in which the gap in the TFT is etched using the source and drain electrodes as the mask. In other words, since the photoresist is not used in the above short-cut process, the photoresist is not left on the semiconductor layer in the portion which will be made into the short-ring. Consequently, the semiconductor layer which is supposed to be made into the short-ring is also etched away when the channel portion (gap) of the TFT is etched.
Therefore, as previously mentioned, to protect the unwanted etching of the semiconductor layer, an additional step is necessary to form a photoresist on the semiconductor layer which will be made into the short-ring before the gap of the TFT is etched. This not only increases the number of the steps in the producing process of the active matrix substrate, but also extends the producing time as well as increasing the manufacturing costs.
It is therefore an object of the present invention to provide an active matrix substrate which can increase a margin for a static electricity and improve the production yield without increasing the number of producing steps, and a producing method of such an active matrix substrate.
To fulfill the above object, an active matrix substrate of the present invention is furnished with:
an insulating substrate;
a plurality of scanning lines and signal lines provided on the insulating substrate in a matrix pattern;
pixel electrodes, each of which being provided to areas enclosed by the scanning lines an signal lines, respectively;
switching elements electrically connected to the scanning lines, signal lines, and pixel electrodes, respectively; and
a resistance control element for electrically connecting at least two lines selected arbitrary from the scanning lines and signal lines, the resistance control element being capable of varying a resistance value thereof under control in response to a voltage applied thereto.
The above active matrix substrate is furnished with the resistance control element which electrically connects at least two lines selected arbitrary from the scanning lines and signal lines and can vary a resistance value thereof under control in response to a voltage applied thereto. Thus, it has become possible to stabilize a resistance between the lines.
If the charges of an external static electricity enter into one of the scanning lines/signal lines connected to the resistance control element, the charges migrate to the other line through the resistance control element. Thus, when the resistance control element is provided between every adjacent lines, the external charges entering into any of the lines can be dispersed to the other lines through the resistance control element in a satisfactory manner.
Consequently, it has become possible to eliminate the static-induced breakdown of the pixel electrodes and switching elements in the active matrix substrate caused by friction while the active matrix substrate is transported or moved, thereby making it possible to increase a margin of the active matrix substrate for the static electricity and improve the production yield.
Also, to fulfill the above object, a producing method of an active matrix substrate of the present invention is composed of the steps of:
forming a first conductive film used as a scanning line material on an insulating substrate;
forming scanning lines, scanning electrodes, scanning electrodes of thin film transistors used as 2-terminal elements by patterning the first conductive film into a predetermined shape;
forming a first insulating layer, a first semiconductor layer, and a second insulating layer sequentially over an area including the scanning lines, scanning electrodes, and the scanning electrodes of the thin film transistors used as the 2-terminal elements;
forming a channel protecting layer by patterning the second insulating film substantially in a same shape as the scanning electrodes and the scanning electrodes of the thin film transistors used as the 2-terminal elements;
forming a second semiconductor layer which will be made into a contact layer over an area including the scanning lines, scanning electrodes, and the scanning electrodes of the thin film transistors used as the 2-terminal elements;
forming channel portions of the thin film transistors and the contact layer by patterning the first and second semiconductor layers into a predetermined shape, respectively;
forming a second conductive film which will be made into signal lines, signal electrodes, drain electrodes, signal electrodes, and drain electrodes of the thin film transistors used as the 2-terminal elements over an area including the contact layer;
forming the signal lines, signal electrodes, drain electrodes, and the signal electrodes and drain electrodes of the thin film transistors used as the 2-terminal elements by patterning the second conductive film into a predetermined shape;
forming a third conductive film which will be made into pixel electrodes; and
forming the pixel electrodes by patterning said third conductive film into a predetermined shape.
According to the above producing method, the 2-terminal elements which control a resistance between the lines and the switching elements which drive the pixel electrodes can be produced concurrently. Therefore, both the 2-terminal elements and switching elements can be composed of the channel etch type thin film transistor. Thus, the producing process does not have to include a separate step to produce the 2-terminal elements that altogether constitute the short-ring for eliminating the static electricity from the switching elements. Consequently, the active matrix substrate having the short-ring can be produced in a shorter time.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.